1. Field of the Invention
The invention relates to methods, systems and structures for heating ice and snow and for modifying ice adhesion strength between ice and selected objects.
2. Statement of the Problem
Ice adhesion to certain surfaces causes many problems. For example, excessive ice accumulation on aircraft wings endangers the plane and its passengers. Ice on ship hulls creates navigational difficulties, the expenditure of additional power to navigate through water and ice, and certain unsafe conditions. The need to scrape ice that forms on automobile windshields is regarded by most adults as a bothersome and recurring chore; and any residual ice risks driver visibility and safety.
Icing and ice adhesion also causes problems with helicopter blades, and with public roads. Billions of dollars are spent on ice and snow removal and control. Ice also adheres to metals, plastics, glasses and ceramics, creating other day-to-day difficulties. Icing on power lines is also problematic. Icing adds weight to the power lines which causes power outages, costing billions of dollars in direct and indirect costs.
In the prior art, methods for dealing with ice adhesion vary, though most techniques involve some form of scraping, melting or breaking. For example, the aircraft industry utilizes a de-icing solution such as ethyl glycol to douse aircraft wings so as to melt the ice thereon. This process is both costly and environmentally hazardous; however, the risk to passenger safety warrants its use. Other aircraft utilize a rubber tube aligned along the front of the aircraft wing, whereby the tube is periodically inflated to break any ice disposed thereon. Still other aircraft redirect jet engine heat onto the wing so as to melt the ice.
These prior art methods have limitations and difficulties. First, prop-propelled aircraft do not have jet engines. Secondly, rubber tubing on the front of aircraft wings is not aerodynamically efficient. Third, de-icing costs are extremely high, at $2500-$3500 per application; and it can be applied up to about ten times per day on some aircraft. With respect to other types of objects, heating heating ice and snow is common. But, heating of some objects is technically impractical. Also, large energy expenditures and complex heating apparati often make heating too expensive.
The above-referenced problems generally derive from the propensity of ice to form on and stick to surfaces. However, ice also creates difficulties in that it has an extremely low coefficient of friction. Each year, for example, ice on the roadway causes numerous automobile accidents, costing both human life and extensive property damage. If automobile tires gripped ice more efficiently, there would likely be fewer accidents.
In certain embodiments of the present invention, electrical energy in the form of a direct current (xe2x80x9cDCxe2x80x9d) bias is applied to the interface between ice and the object that the ice covers. As a result, the ice adhesion strength of the ice to the surface of the object is decreased, maing it possible to remove ice from the object by wind pressure, buffeting or light manual brushing.
Other above-referenced problems would be lessened if the ice adhesion strength between ice and surfaces of objects in contact with the ice were increased. For example, if the ice adhesion strength were increased between automobile tires and icy roadways, then there would be less slippage and fewer accidents.
If a charge is generated at the interface of ice in contact with a object, it is possible to selectively modify the adhesion between the ice and the object.
In one aspect, the invention provides a power source connected to apply a DC voltage across the interface between ice and the surface upon which the ice forms. By way of example, the object having the conductive surface can be an aircraft wing or a ship""s hull (or even the paint applied to the structure). A first electrode connects with the surface; a nonconductive or electrically insulating material is applied as a grid over the surface; and a second electrode is formed by applying a conductive material, for example conductive paint, over the insulating material, but without contacting the surface. The surface area of the second electrode should be small as compared to the overall surface area protected by the system. By way of example, the surface area of the object being protected (i.e., that area sought to be xe2x80x9cice-freexe2x80x9d) should be at least about ten times larger than the surface area of the second electrode.
One or more wires connect the second electrode to the power source; while one or more wires connect the first electrode to the power source. Ice forming over the surface and the conductive grid second electrode completes the circuit. A voltage is then applied to the circuit, selectively, which controllably modifies the ice adhesion strength of the ice with the object.
A voltage regulator subsystem also preferably connects with the circuit so as to adjustably control the voltage applied across the interface and so as to achieve control over the ice adhesion strength. Ice made from different concentrations of ions can modify the optimum voltage for which the ice adhesion strength is at a minimum; and the voltage regulator subsystem thereby provides a mechanism by which the minimum can be changed selectively.
Other subsystems preferably connect with the circuit to provide other features, for example to detect whether water or ice completes the circuit. In one aspect, the power source is a DC supply (e.g., a battery) which provides voltage to the circuit and which connects to the deicing electrodes. In another aspect, a DC ammeter connects with the circuit to measure the DC conductivity of the ice (i.e., the semi-conductive layer which xe2x80x9cshortsxe2x80x9d the two electrodes when formed over the surface and any part of the grid second electrode). In another aspect, an AC supply connects with the circuit to generate AC voltages between about 10 kHz and 100 kHz, selectively. According to another aspect, an AC ammeter also connects with the circuit to measure the AC conductivity of the ice at frequencies within the 10-100 kHz range. In still another aspect, a current comparator compares the AC and DC conductivities.
These aspects thus provide circuitry which can, for example, distinguish whether the semi-conductive layer formed over the surface is ice, which might be dangerous, or surface water. The AC conductivity (in the above-mentioned range) and DC conductivity of water are substantially the same. With respect to ice, however, the AC conductivity and DC conductivity differ by two to three orders of magnitude. This difference in conductivity is measured by the respective ammeters and is compared in the current comparator. When the difference in conductivity is greater than a predetermined set point, the current comparator signals an icing alarm. At this point, for example, the voltage regulator subsystem can operate to apply a DC bias to the circuitxe2x80x94and thus to the interfacexe2x80x94at a desired field strength which sufficiently reduces the ice adhesion strength. According to one aspect of the invention, when ice is detected on an aircraft wing, the icing alarm initiates a feedback loop within the system which (a) measures ice conductivities, (b) determines appropriate bias voltages to reach minimum (or near minimum) ice adhesion conditions, and (c) applies a bias voltage to the ice-wing interface to facilitate ice removal.
Those skilled in the art should appreciate that the above-described system can be applied to surfaces of many objects where it is desired to reduce ice adhesion strength, such as on car windshields, ship hulls and power lines. In such cases, if the surface of the object is weakly conductive, it is desirable to xe2x80x9cdopexe2x80x9d the surface of the object such that it is sufficiently conductive. Doping techniques are known to those in the art. Automobile tires, for example, can be doped with iodine to make the rubber conductive. Automobile glass, likewise, can be doped with either ITO or fluoride doped SnO2 to make the windshield an acceptable semiconductor.
In preferred embodiments in accordance with the invention, the ice is doped with a salt, which increases the electrical conductivity of the ice for the DC bias and enhances the effect of the DC bias on the adhesion strength. Preferably, doping is accomplished by applying a porous material to the surface of the object to be protected. The porous material is impregnated with salt, and then the porous material releases small amounts of salt into the ice when it forms. Also, the object being protected against ice and snow may be doped in order to increase the electrical conductivity of the object, especially at its surface.
In another embodiment in accordance with the invention, a self-assembling monolayer (xe2x80x9cSAMxe2x80x9d) is applied to the surface of the object to be protected. The material forming the SAM is selected to have high adhesion to the surface. The SAM material is also selected to be either hydrophobic or hydrophilic, to decrease or increase ice adhesion strength.
In another aspect, the invention provides a system and method for deicing power lines and other objects. The surface of the object, such as power lines, is coated with a material that absorbs electromagnetic energy. The absorption of the electromagnetic energy heats the power lines above the ice""s melting point, thus melting the ice. The coating material may be a ferroelectric, a semi-conductor, or a ferromagnetic material. In one embodiment, the coating has properties that depend on a specific temperature based on either the dielectric or magnetic loss. These properties cause the absorption of electromagnetic energy and the resulting heating of the wires only when the ambient temperature drops below the ice""s melting point. Alternatively, the coating material could be turned xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d if a separate power supply is used to heat the coating.
In a particular variation, ice itself is utilized as a lossy dielectric coating at high frequency, such as at 60 kHz. Further, skin-effect heating at high frequency may be utilized to melt ice and snow on power lines.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.